An important goal of photography is to capture and reproduce the visual richness of a real environment. Lighting is an integral aspect of this visual richness and often sets the mood or atmosphere in the photograph. The subtlest nuances are often found in low-light conditions. For example, the dim, orange hue of a candlelit restaurant can evoke an intimate mood, while the pale blue cast of moonlight can evoke a cool atmosphere of mystery.
When capturing the natural ambient illumination in such low-light environments, photographers face a dilemma. One option is to set a long exposure time so that the camera can collect enough light to produce a visible image. However, camera shake or scene motion during such long exposures will result in motion blur. Another option is to open the aperture to let in more light. However, this approach reduces depth of field and is limited by the size of the lens. The third option is to increase the camera's gain, which is controlled by the ISO setting. However, when exposure times are short, the camera cannot capture enough light to accurately estimate the color at each pixel, and thus visible image noise increases significantly.
Flash photography was invented to circumvent these problems. By adding artificial light to nearby objects in the scene, cameras with flash can use shorter exposure times, smaller apertures, and less sensor gain and still capture enough light to produce relatively sharp, noise-free images. Brighter images have a greater signal-to-noise ratio and can therefore resolve detail that would be hidden in the noise in an image acquired under ambient illumination. Moreover, the flash can enhance surface detail by illuminating surfaces with a crisp point light source. Finally, if one desires a white-balanced image, the known flash color greatly simplifies this task.
As photographers know, however, the use of flash can also have a negative impact on the lighting characteristics of the environment. Objects near the camera are disproportionately brightened, and the mood evoked by ambient illumination may be destroyed. In addition, the flash may introduce unwanted artifacts such as red eye, harsh shadows, and specularities, none of which are part of the natural scene. Despite these drawbacks, many amateur photographers use flash in low-light environments, and consequently, these snapshots rarely depict the true ambient illumination of such scenes.